FERTILISATION 53 



product of hydrolysis (B) whose formation is stimulated by acid, and a 

 synthesised substance (5), the concentration of which is increased by 

 hypotonicity. 



(5) Bataillon, like Loeb, emphasised the double nature of the process. 

 In his view one element is the production of a cortical change, and the 

 second the production of the cleavage apparatus in the interior of the 

 egg- 

 Recent discussions of the pros and cons of these various theories may 

 be found in Dalcq (1928), Tyler (1941) and Brachet (i944)- The fact that 

 there are so many and such different theories still in the field indicates that 

 none of them is very satisfactory. The process of activation must almost 

 certainly be complex and involve at least the two factors emphasised by 

 Bataillon; that is to say, an action on the cortex and an action on the 

 internal cytoplasm. Little more need be said at present about the cortical 

 effect than has been given above in the discussion of normal fertiHsation. 

 It may be mentioned, however, that it is not uncommon for partheno- 

 genetically activated eggs to fail to produce a plane of bilateral symmetry, 

 but to develop into radially symmetrical forms. This is another demon- 

 stration that, in the species in which this happens, the point of entry of 

 the sperm plays an essential role in determining the dorso-ventral plane. 

 The failure of this plane to appear in parthenogenesis is presumably a 

 consequence of the fact that the activating agent in this case operates 

 simultaneously over the whole surface of the egg. 



Several very interesting facts have emerged in recent years about the 

 internal cytoplasmic events in parthenogenetically activated eggs. In the 

 normal development of almost all types of animal eggs the spindles for the 

 cleavage divisions arise from centrosomes which have been brought in 

 by the sperm. The centrosome remaining in the egg from the last matura- 

 tion division normally degenerates and takes no part in the later cleavages. 

 We have to inquire, therefore, whence the centrosomes for the cleavage 

 spindles come in cases of parthenogenesis when no sperm centrosome is 

 available. 



In some forms there is no doubt that the spindles wliich arise in con- 

 nection with the formation of the polar bodies can in these circumstances 

 take over the control of the cleavage division. A case which 4ias been 

 studied in detail is that of the echiuroid worm Urechis (Tyler 1941, Fig. 

 3.4). The egg of this form when laid contains a large germinal vesicle, and 

 at the animal pole there is a deep indentation of the surface. Treatments 

 either with hypotonic solutions or with ammoniacal sea-water can bring 

 about activation, which is normally made visible by the rounding-up of 



